Definition
To remain completely silent; to not utter a word.
When asked about the issue, he didn't say boo.
She didn't say boo during the meeting, just listened quietly.
He didn't say boo all evening, as if he was sulking.
To be very timid and easily frightened; to be excessively nervous.
The little boy is so timid that he doesn’t say boo when he sees even the slightest movement.
She is very nervous and doesn't say boo when she is in public.
I know someone who is so afraid that she doesn't say boo when facing any problem.